Posted: Thu Oct 29, 2015 5:03 pm Post subject: impact of tuning an antenna on its original surface current

Hello
I have simulated an antenna and get its J surface current and H near field.
Since this antenna is really unmatched w.r.t a 50 ohms cable, I have used afterwards a LC tuning circuit. It is now well tuned but its surface current and H near field are strongly impacted. Their distributions are totally differents (null and maximum at different locations) from the previous ones. Is it normal?
I did expect the same repartition but with a greater antenna current since the reflected power at the input antenna terminals is now lower.
However it is not the case.
Is this a normal situation? Or maybe I have badly simulated this antenna?

This is an interesting question from a theoretical perspective. ...From a practical perspective, I'm curious what code you're using and how exactly you're simulating the matching network. A common approach would be to simulate the antenna as a "transmitter" and export the input impedance as S-parameters for use in another circuit code. Maybe with "co-simulation" in a code like ADS or CST you're seeing the effect on the currents.

From a theoretical perspective, I think the dependence of the currents and fields on the matching network is OK. Consider a receiving antenna with an arbitrary terminating impedance. Within your simulation volume, the ratio of the line integral of the E-field along the feed and the contour integral of the H-field around the feed must give the terminating impedance. As the termination impedance changes, the ratio of these field integrals will change. It's a pretty safe bet that if the ratio of the field integrals must change, then the fields themselves must change.

Now -- you're mentioning seeing the shapes of the currents and fields away from the feed change with the matching network. Again, consider the receiving antenna case. If you regard space as a spherical transmission line, with incoming and outgoing spherical waves (e.g., Chu's "modes of free space"). In a way analogous to a regular transmission line, due to the reflected wave, you'll see current/H-field and voltage/E-field maxima and minima move around depending on the terminating impedance (this is how a slotted line works).

OK, so this I feel answers your question for the receiving case. It's perhaps a little harder to see for the transmitting case, which is probably the way you are running your code... I'll have to think about that more to come up with an argument for that.

Thank you very much for your reply. It is very interesting, especially the concept of integrating the E and H fields.
My (very) basic explanation was:
since I am using an ideal voltage source, tuning the antenna means changing its input impedance and hence only the input current (in terms of the magnitude and phase). So I was assuming the current is also changing elsewhere... But it is a very basic explanation...